A typical integrated circuit package includes an integrated circuit die attached to a leadframe, the leadframe being the backbone of a typical molded plastic package. Leadframes serve first as a die support fixture during the assembly process, and are subsequently electrically connected to the die bond pads after die-attach by wirebonding. After molding, the leadframe becomes an integral part of the package. The package includes external terminals (e.g., leadfingers) for power and signal distribution. In addition, the package may provide for heat dissipation.
As is well known in the art, high-resistance and thermally inefficient integrated circuit devices significantly reduce system reliability and electrical efficiency. One of the factors contributing to such inefficiency involves the use of wirebond connections between the package's leadfingers and the die. Such wirebonds contribute a large percentage of the overall package resistance. In addition, wirebond cratering and a so-called "purple plague" phenomena associated with wirebonds affect device reliability. "Purple plague" is a phenomenon when gold (the typical material for bond wires) is combined with an aluminum top metal on the die. When gold is combined with an aluminum top metal, intermetallic formations may result, which can cause open circuits.
Another contributing factor of device inefficiency is inadequate heat dissipation. For example, heat from a power MOSFET (Metal-Oxide-Semiconductor Field-Effect-Transistor) device on a printed circuit board (PCB) typically is removed through a plate that forms the drain-lead connection on the back of the die. The plate conducts heat to the PCB through a conductive connection. With smaller devices and higher power, such heat dissipation may not be adequate in some applications.
To improve device resistance and heat dissipation in power MOSFET applications while keeping a small package size, a copper strap has been developed to eliminate the wirebonds associated with the source inputs of the power MOSFET device. By replacing the wirebonds connecting the source to the leadframe with a solid copper strap, a highly conductive (both thermally and electrically) path between the leadframe and the die is created. The reduction in thermal and electrical resistance allows for, e.g., less paralleling of devices, smaller chips and package outlines and higher reliability. Circumventing wirebonding also allows a decrease in assembly time and elimination of the cratering and purple plague phenomena associated with wirebonds.
In addition, the copper strap creates a low-resistance parallel path with the die's top surface, further reducing package resistance. Heat dissipation also is improved because the copper strap provides two thermal-dissipation avenues - one through the source leadfingers to the PC board and the other through a top surface of the package, since the copper strap is routed close to the top of the mold compound encapsulant and thus facilitates heat radiation from the top surface of the package.
FIG. 1 illustrates a flowchart of a conventional assembly process for a power MOSFET package employing a copper strap. FIGS. 2, 3A, 3B, 4A, 4B, 5A and 5B show various stages during the assembly process. The process starts in Step 10.
Referring to FIGS. 1 and 2, Step 12 loads a leadframe 50 into a typical die attach machine. Step 14 dispenses a conductive adhesive epoxy 56 onto a leadframe pad 52 of leadframe 50. Step 16 picks a die 58 from a wafer tape (not shown) and places die 58 on top of conductive epoxy 56 on leadframe pad 52 (FIGS. 3A and 3B) using a conventional method. Step 18 loads the die-bonded leadframe strips into transport magazines and then unloads the transport magazines from the die attach machine. Step 20 loads the transport magazines into a conventional curing oven for a first curing step, wherein conductive epoxy 56 is cured.
Subsequent to curing conductive epoxy 56, Step 22 of FIG. 1 removes the magazines of die bonded leadframe strips from the oven and loads the leadframes into a solder paste dispensing system. Referring to FIGS. 4A and 4B, Step 24 dispenses a solder paste 60 onto a top surface of die 58 and input leadfingers 54. Step 26 of FIG. 1 singulates a copper strap 62 from a reel or a matrix reel. The copper strap 62 is then picked and placed onto the solder paste 60 on die 58 and input leadfingers 54. FIGS. 5A and 5B show top and side views of copper strap 62 on die 58 and input leadfingers 54.
Step 28 of FIG. 1 loads the leadframe units into a reflow oven for a second curing step wherein solder paste 60 is cured. Referring to FIG. 5B, a problem associated with Step 28 is that, after solder paste 60 curing, the bond line thickness BLT1 at the bottom of copper strap 62 is uncontrolled. The leadframe units are then placed into another transport magazine in preparation for subsequent steps. Once the transport magazine is full, the units are unloaded from the oven in Step 30. The process ends in Step 32.
The process of FIG. 1 requires the use of both an epoxy and solder paste, and accordingly requires two curing steps, namely, an epoxy cure for die attach and a solder paste cure for copper strap attach. Artisans will appreciate that the cost of packaging an integrated circuit die depends, in part, on the materials used and the number of steps of the packaging process.
Therefore, what is needed is a less costly and more efficient method of packaging an integrated circuit die. In particular, what is needed is a more efficient method of attaching a conductive strap that allows electrical connection between a leadframe and an integrated circuit die.